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The Concept of a Virtual Server

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


The concept of a virtual server is a very significant feature in a failover cluster that allows smooth and seamless connectivity to clustered computing resources or services by client applications.

To users and clients, connecting to a database or service running as a clustered virtual server appears to be the same as connecting to a single, physical server. In fact, any node in the cluster, but usually the primary or surviving node, can host the connection to a virtual server. The user or client application will not know which node is actually hosting the virtual server. As shown in the Figure 3.8, the Virtual Server is an addressing mechanism for the client connections.

Figure 3.8: Cluster Access by using Virtual Server

Server clusters manage the virtual server as a resource that is mapped to the IP address or network name. Application client connections to a virtual server are made by a client session that knows only the IP address that the cluster service publishes as the address of the virtual server. The client view is simply a view of individual network names and IP addresses.

For example, in a typical database cluster having two physical nodes, each with distinct IP addresses, the cluster server configures an additional IP address, usually termed as virtual server or host. This virtual host always maps to the active node in the cluster where database service is provided.

Failover Process

Whenever there is a failure in the primary node, and the application or database is in disabled function, the failover process begins as initiated by a script or agent. Fail-over in host-based database system usually includes the following steps in sequential order.

* Detecting failure by monitoring the heartbeat and checking the status of resources.

* Reorganizing cluster membership in the Cluster Manager.

* Transferring disk ownership from the primary node to a secondary node.

* Mount the file system on secondary node.

* Starting database instance on secondary node.

* Recover the database and rollback uncommitted data.

* Reestablishing client connections to the fail over node (database)

The following are examples of how a typical resource group is configured in a Veritas Cluster server and Microsoft Cluster service.

Examples:

Veritas Cluster Server (VCS), Service Group is the basic unit of fail over. Service group fails over to backup node when failure occurs at the primary node. Service groups consist of related resources that work together to deliver database service to clients. Service Groups allow the monitoring and controlling of service availability as a whole, as opposed to the individual items (servers, disks, software, etc.) The failure of one critical item in the service group will cause the entire group to fail over to another system.

In Oracle database implementation within the framework of Microsoft Cluster Service (MSC), the Cluster Group includes the following resources:

One or more virtual addresses, each of which consists of an IP address and network name:

* The Oracle database server

* All disks used by the Oracle database

* A Net8 (or SQL*Net) network listener that listens on the virtual address (or addresses) of the group for connection requests to the databases in the group

* An Oracle Intelligent Agent configured to use one of the group?s virtual addresses (if Oracle Enterprise Manager will be used to manage the database)

The Cluster group is the basic failover unit in case of MSC. Oracle provides failsafe manager tools to configure and manage Oracle database service failover with in MSC framework.

The above examples demonstrate that the database instance has been freshly started after required resources are online. It is a mutually exclusive condition in which the database instance resides on a primary node or on a backup node.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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